In order to understand the mechanisms underlying tumor development, it is important to distinguish among the toxic response an organism makes to a chemical carcinogen and those tissue changes which are necessary for the formation of a tumor. Mice are prone to develop a specific kind of lung tumor as they age, namely, an adenoma. Various carcinogens can hasten the number of tumors formed and the age when they appear. Among the biochemical responses of the lung to urethane are alterations in DNA synthesis and the phosphorylation of protein. The possible roles of these reactions in the response to urethane will be examined as follows. The food additive, butylated hydroxytoluene (BHT), can modulate the number of tumors formed in response to urethane. By itself, BHT can also cause lung injury. Another chemical, cedrene, can prevent BHT-caused lung injury. Thus by various sequences of injections of agents, the number of tumors can be modified without altering the amount of urethane injected. Finally, different inbred strains of mice vary in their adenoma responsiveness to urethane. DNA synthesis and protein phosphorylation will be examined in different strains after urethane injection. The consequent findings should increase our knowledge of the events required for adenoma formation.